The present invention relates to tip shrouded turbine buckets and, in particular, to the reinforcement of the tip shroud to stiffen the shroud and enhance creep life.
Gas turbine buckets or blades are airfoil-shaped components designed to convert the thermal and kinetic energy of flowpath gases into mechanical rotation of the rotor. Turbine performance may be enhanced by providing a seal at the tip of the airfoil to block the flow of air over the top of the airfoil which would otherwise bypass the airfoil and thus not perform any work on the rotor. Thus, such tip seals close the gap between the bucket and the surrounding stationary casings. A variety of seal designs have been developed, all of which require the introduction of a tip shroud to the tip of the airfoil.
A typical tip shrouded turbine bucket configuration is illustrated by way of example in FIGS. 1-5. With reference thereto, a typical gas turbine bucket 10 includes a bucket airfoil 12. This is the active component that intercepts the flow of gases, acting as a windmill vane to convert the energy of the gases into tangential motion, which in turn rotates the rotor to which the buckets are attached. At the top of the airfoil, a seal rail 14 is provided to prevent the passage of flowpath gases through the gap between the bucket tip and the inner surface of the surrounding stationary components (not shown). The seal rail 14 extends circumferentially around the bucket row, beyond the airfoil 12 sufficiently to match up with the seal rails provided at the tip of adjacent buckets, effectively blocking flow from bypassing the bucket row so that airflow must be directed to the working length of the bucket airfoil 12. The tip shroud 16 is added to provide lateral support to the connection also provides a vibrational constraint to the buckets, raising the buckets' natural frequencies and helping to prevent resonance failures.
The tip shroud 16 is essentially a flat plate supported towards its center by the airfoil 12 but subject to high temperatures and centrifugal loads. As a result, material creep becomes a concern. As noted above, the placement of the tip shroud 16 over the airfoil tip allows the airfoil 12 itself to support some of the shroud directly. However, as can be seen from the schematic illustration of FIG. 6, the corners of the shroud are relatively unsupported. Some designs allow for the positioning of the shroud to allow corners corresponding to corners 18 and 26 to be essentially directly over the airfoil 12 but with the consequence that corners 20 and 24 are left quite unsupported. The stiffness of the shroud plate (16) is enhanced by the presence of the seal rail 14. As mentioned above, the shroud is shaped such that the aerodynamic loading on the airfoil causes adjacent shrouds to lock together providing a built in boundary condition for the bucket tip and raising a number of bucket vibrational modes out of the machine operating range. However, creep deformation of the corners of the shroud may result in this locking being lost, lowering the bucket response frequencies, potentially resulting in airfoil high cycle fatigue (HCF) failures.
There have been a number of prior approaches to reduce the problems of shroud creep. For example, the use of directionally solidified or single crystal alloy to enhance creep resistance of the airfoil has been proposed. The complex geometry and tight radii associated with the tip shroud region of the design, however, has prevented bucket casting technology from successfully introducing these features into the tip shroud. Another approach is cooling the shroud. Airfoils themselves may be cooled with, e.g., cooling air introduced at the bucket inner attachment and circulated through the airfoil either through radial passages that are open to the tip of the airfoil or through cast serpentine passages in the airfoil. Again, however, the complexity and size of the tip shroud generally precludes manufacture of such holes in the shrouds, thus preventing active cooling of the shrouds. Finally, modifications to the mechanical design of the shrouds have been considered. Specifically, the shrouds may be shaped or scalloped to reduce centrifugal loads on the corners. This approach has met with significant success but there are limits to its effectiveness, and growth engines will require further enhancements.